S11 S12 S13

The three roots of Equation 1.12, herein denoted as ffP1, ffP2, and cP3, are the principal stresses acting on the three orthogonal planes. The components of a unit vector that defines the principal plane (i.e., n1Pi, n2Pi, n3Pi) corresponding to a specific principal stress sPi (with i = 1, 2, 3) can be evaluated using any two of the following equations: